It is frequently necessary to determine whether a particular component, line, cable, element, etc., in an electrical circuit is "live," that is, whether it bears a voltage. Such a determination is especially important where the voltage of concern is substantial, for obvious safety reasons. Even where electrical circuits proper are not involved, the ability to detect a voltage may be important. To cite but a few examples: because of the fire hazard represented by sparks, it is often desirable to know if there is an accumulation of static electricity in an object. A piece of equipment associated with an electrical circuit but not part of the circuit itself may need to be tested to determine if it has been properly grounded of if it has been unintentionally electrically connected to the circuit. A capacitor may need to be tested to determine if it is charged. Hereinafter, the generic term "electrical component" will be used to designate not only components, lines, cables, etc. in an electrical circuit proper, but also articles and equipment in which a static or other electrical charge is to be detected.
Preferably, the device for such determinations is rugged, portable, easy to use, reliable, and inexpensive. Ruggedness permits the device to survive adverse use conditions in which it might be subjected to impact, temperature extremes, chemicals, etc. Portability is advantageous because a portable device can monitor a multiplicity of components instead of being permanently or semi-permanently attached to a particular one. For example, where a single electrical system requires non-simultaneous voltage monitoring at several different points, this can be done with a single portable device. An easy-to-use device does not require complex physical or electrical connections to the component being monitored. Preferably, the connections are simple to make and disconnect; more preferably, the device operates remotely--that is, it requires no connections whatsoever. A remote device is also advantageous safety-wise, since the danger of an electric shock in the event of a malfunction or misuse is reduced. The device should be reliable, for a false negative reading can create a severe risk of injury. The economic advantages of an inexpensive device are self-evident. For example, where it is desirable to monitor several points in an electrical system simultaneously, an inexpensive device reduces or minimizes the cost of installing one such device at each point.
Further, while for certain applications the device need only display a qualitative, "on" or "off" indication of voltage, for other applications it is desirable or essential that the device provide a quantitative or semi-quantitative read-out of the voltage being detected.
Liquid crystals have been employed in the display element of many devices, including voltage detecting devices. Such devices rely on the preferential alignment of the long axes of the constituent molecules along one direction, a phenomenon known as molecular anisotropy, and the ability to control this preferential alignment with an applied voltage. The optical properties of such liquid crystals are also anisotropic. For example, along one axis, light may be scattered or absorbed, giving the liquid crystal material an opaque or dark appearance, while along a perpendicular axis, light may be transmitted, giving the liquid crystal material a transparent appearance.
Liquid crystal displays (LCD's) possess numerous advantages which are reflected by their popularity and ascendancy over other types of displays, such as light emitting diodes, or LED's. LCD's are easily fabricated into complex patterns which can be in the form of alphanumeric characters or graphic symbols. A single LCD display can comprise multiple elements which can be simultaneously or independently addressed, so that a single display panel can convey different messages. Witness, for example, the familiar seven bar display of LCD watches and calculators. LCD's consume very little electrical power, and the threshold voltage needed to switch from one state to the other is relatively low.
A number of LCD devices for detecting a voltage are known. But each has one or more disadvantages limiting its utility.
U.S. Pat. No. 3,627,408 (Fergason) discloses a device for detecting an unknown electric field, comprising a layer of cholesteric liquid crystalline material and a first and a second electrical conductive members for impressing an electric field across the layer. A voltage source is electrically connected to the first and second conductive members.
DE No. 2,949,561-A1 (Walter) discloses an analog voltage display in which the active display area comprises a number of individual LCD elements formed from the overlap of each electrode with two opposing electrodes. The extent of overlap is varied so that a series of LCD units of increasing size is formed. The display is electrically connected across the voltage source being estimated.
DE No. 3,219,703-A1 (Kehr) discloses an electrical connector for medium and high voltage cables in which an LCD monitor on the connector gives a continuous read out of on/off state of the cable. The monitor is coupled via a voltage reduction circuit to the current carrying parts of the cable. The monitor is connected to the shield of the cable, which is grounded.
DE No. 3,308,972-A1 (Heverhagen et al.) discloses a device for displaying the operating state of a fuse. An LCD with a series-connected capacitor is electrically connected across the fuse being monitored. Both connecting leads may be ohmically connected, or one may be capacitively coupled while the other is ohmically connected.
DE No. 3,402,655-A1 (Goehlich) discloses a device for indicating the state of a high voltage installation. This device, which has an LCD display, is electrically connected to the installation by a single-pole connecting line and is capacitively coupled to ground.
Each of these prior art devices is limited in that at least one of the device leads must be ohmically connected to the electrical component being tested or to ground. Some of the devices, such as those of Fergason, Heverhagen, Kehr, and Goehlich, can provide an "on/off" reading, but not a quantitative estimation of voltage. These prior art devices, because of the requirement for ohmic contact with the electrical component or ground, are unsuitable for detecting a buildup of static electricity, which is readily discharged upon contact with the device.